MEIOSIS RS:Ms. Alvarez CT:Mrs. Rojas

Two different types of cells are found in sexually reproducing organisms Somatic Cells: are “body” cells that have a normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. They divide through mitosis. Germ cells: are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova(egg) are gametes. They divide through meiosis.

Meiosis Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material. Meiosis involves one round of DNA replication and two successive nuclear divisions.

How sex cells are produced n=23 sperm haploid (n) meiosis II Secondary Spermatocyte 2n=46 human sex cell diploid (2n) n=23 meiosis I Primary Spermatocyte Secondary Spermatocyte 4 sperm cells are produced from each primary spermatocyte.

Fertilization n=23 egg 2n=46 zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Chromosomes Diploid organisms (2n) have two matching homologues chromosomes. One from mom (mom’s DNA).… the other from the dad (dad’s DNA). Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell).

Homologous Chromosomes (because a homologous pair consists of 4 chromatids it is called a “Tetrad”) eye color locus hair color centromere Paternal Maternal

During Prophase I “Crossing Over” occurs. Crossing Over is one of the Two major occurrences of Meiosis During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over.

Crossing Over Occurs During Prophase I creates variation (diversity) in the offspring’s traits. Tetrad nonsister chromatids chiasmata: site of crossing over segments of nonsister chromatids break and reattach to the other chromatid variation

Question: 10 chromosomes (haploid) 10 chromosomes (haploid) A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 10 chromosomes (haploid) 10 chromosomes (haploid)